The present invention relates to an oxynitride phosphor activated by a rare earth element, which makes high luminance of a white light emitting diode (white LED) employing a blue light emitting diode (blue LED) as a light source possible. Further, the present invention relates to a sialon type phosphor optically activated by a rare earth element, which makes high luminance of white LED employing blue LED or an ultraviolet emitting diode (ultraviolet LED) as a light source possible.
Phosphors are widely known wherein a silicate, a phosphate (such as apatite) or an aluminate is used as a matrix material and such a matrix material is activated by a transition metal or a rare earth metal. On the other hand, phosphors wherein a nitride or an oxynitride is used as a matrix material and such a matrix material is activated by a transition metal or a rare earth metal, are not well known.
With respect to nitride phosphors, for example, German Patent 789,890 discloses aluminum nitride activated by manganese, and a literature xe2x80x9cIzv. Akad. Nauk SSSR, Neorg. Masterxe2x80x9d 17(8), 1431-5 (1981) discloses magnesium silicon nitride (MgSiN2) activated by a rare earth element. Recently, only a red-emitting phosphor having ZnSiN2 having a distorted wurtzite structure activated by Mn (T. Endo et al. xe2x80x9cHigh pressure synthesis of xe2x80x9cperiodic compoundxe2x80x9d and its optical and electrical propertiesxe2x80x9d, In T. Tsumura, M. Doyama and Seno (Editors), New Functionality Materials, Volume C, Elsevier, Amsterdam, The Netherlands, pp. 107-112(1993)), a red-emitting phosphor having CaSiN2 activated by Eu (S. S. Lee et al. xe2x80x9cPhotoluminescence and Electroluminescence Characteristic of CaSiN2:Euxe2x80x9d, Proc. SPIE-Int. Soc. Opt. Eng., 3241, 75-83(1997)) and a phosphor having Ba2Si5N8 activated by Eu, have been reported.
With respect to oxynitride phosphors, a phosphor using xcex2-sialon as the matrix material (JP-A-60-206889), a phosphor having a silicate mineral or a Yxe2x80x94Sixe2x80x94Oxe2x80x94N type composite silicon oxynitride having an apatite structure activated by Ce (J. W. H. van Krevel et al. xe2x80x9cLong wavelength Ce3+ emission in Yxe2x80x94Sixe2x80x94Oxe2x80x94N materialsxe2x80x9d, J. Alloys and Compounds, 268, 272-277(1998)), a Ba1xe2x88x92xEuxAl11O16N phosphor having a xcex2-alumina structure (H. Hintzen et al. xe2x80x9cOn the Existence of Europium Aluminum Oxynitrides with a Magnetoplumbite or xcex2-Alumina-Type Structurexe2x80x9d, J. Solid State Chem., 142, 48-50(1999), and S. R. Jansen et al. xe2x80x9cEu-Doped Barium Aluminum Oxynitride with xcex2-Alumina-Type Structure as New Blue-Emitting Phosphorxe2x80x9d, J. Electrochem. Soc., 146, 800-806(1999)) have been reported. Recently, only a phosphor using an oxynitride glass as a matrix material, has been proposed (JP-A-2001-214162).
Whereas, white LED has been used, for example, in the field where reliability is required for e.g. emergency illumination or signal light, in the field where miniaturization and weight reduction are desired, for example, for in-vehicle lightening or liquid crystal backlight, or in the field where visibility is required for e.g. guide plates at railway stations. The emitted color of such white LED, i.e. white light, is obtained by color mixing of lights and is one obtained by mixing of blue light emitted by blue LED of InGaN type with a wavelength of from 450 to 550 nm as a light source and yellow light emitted from the phosphor.
As a phosphor suitable for such white LED, a phosphor having Ce doped to a YAG type oxide represented by the composition formula (Y,Gd)3(Al,Ga)5O12, is most commonly employed. This phosphor is applied as a thin coating on the surface of the above-mentioned InGaN type blue LED as a light source.
However, an oxide type phosphor usually has a drawback that the emission intensity substantially decreases if the excitation wavelength exceeds 400 nm. Accordingly, white LED obtained by coating the surface of a blue LED chip with a phosphor made of a YAG type oxide, has been considered to have a difficulty such that the excitation energy of the YAG type oxide as the phosphor does not agree to the excitation energy of the blue LED as the light source, whereby the excitation energy can not efficiently be converted, and it is difficult to prepare white LED having high luminance.
In a first aspect, the present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an oxynitride phosphor activated by a rare earth element, which makes high luminance of a white light emitting diode (white LED) employing a blue light emitting diode (blue LED) as a light source possible.
The inventors of the present invention have found that the positions of excitation/emission peaks shown by a conventional oxide type phosphor shift towards a long wavelength side, when oxygen atoms surrounding the rare earth element as the emission center are substituted by nitrogen atoms to reduce the influence which electrons of the rare earth element receive from the surrounding atoms, and on the basis of this technical finding, they have proposed a phosphor which employs an oxynitride glass as the matrix material and which has an excitation spectrum extending to a visible region (xe2x89xa6500 xcexcm).
On the basis of the above technical finding, the present inventors have further studied the presence of another oxynitride phosphor and as a result, have found that a crystalline oxynitride phosphor employing xcex1-sialon having a higher nitrogen content than the oxynitride glass, as the matrix material, wherein a part or all of metal Me (where Me is at least one metal selected from the group consisting of Ca, Mg, Y and lanthanide metals excluding La and Ce) in xcex1-sialon solid solution as the matrix material, is substituted by lanthanide metal Re1 (Re1 is at least one metal selected from the group consisting of Ce, Pr, Eu, Tb, Yb and Eu), or two lanthanide metals Re1 and a coactivator Re2 (where Re2 is Dy), to be an emission center, makes high luminance white LED possible. Thus, the first aspect of the present invention has been accomplished on the basis of this discovery.
Thus, according to the first aspect, the present invention provides:
1. An oxynitride phosphor activated by a rare earth element, represented by the formula MexSi12xe2x88x92(m+n)Al(m+n)OnN16xe2x88x92n:Re1yRe2z, wherein a part or all of metal Me (where Me is at least one metal selected from the group consisting of Ca, Mg, Y and lanthanide metals excluding La and Ce) in xcex1-sialon solid solution, is substituted by lanthanide metal Re1 (where Re1 is at least one metal selected selected from the group consisting of Ce, Pr, Tb, Yb and Er), or two lanthanide metals of Re1 and a coactivator Re2 (where Re2 is Dy), to be an emission center.
2. The oxynitride phosphor activated by a rare earth element according to Item 1, wherein when metal Me is bivalent, 0.6 less than m less than 3.0 and 0xe2x89xa6n less than 1.5.
3. The oxynitride phosphor activated by a rare earth element according to Item 1, wherein when metal Me is trivalent, 0.9 less than m less than 4.5 and 0xe2x89xa6n less than 1.5.
4. The oxynitride phosphor activated by a rare earth element according to any one of Items 1 to 3, wherein m=1.5, n=0.75 and in the composition formula MexSi9.75Al2.25O0.75N15.25:Re1yRe2z, 0.3 less than x+y less than 0.75 and 0.01 less than y+z less than 0.7 (where y greater than 0.01 and 0.0xe2x89xa6z less than 0.1).
5. The oxynitride phosphor activated by a rare earth element according to Item 4, wherein 0.3 less than x+y+z less than 1.5, 0.01 less than y less than 0.7 and 0xe2x89xa6z less than 0.1.
6. The oxynitride phosphor activated by a rare earth element according to Item 2, 4 or 5, wherein metal Me is Ca.
The above oxynitride phosphor is composed of a single phase of xcex1-sialon, whereby it is required to incorporate a large amount of the rare earth metal, which limits reduction of costs.
In a second aspect, the present invention has been made under such circumstances, and it is another object of the present invention to provide a sialon type phosphor comprising xcex1-sialon dissolving a rare earth element in the structure, xcex1-sialon and unreacted silicon nitride, which makes high luminance of a white light emitting diode (white LED) employing a blue light emitting diode (blue LED) as a light source possible.
Thus, in the second aspect, the present invention provides:
7. A sialon type phosphor in the form of a powder comprising at least 40 wt % of xcex1-sialon represented by the formula (Cax,My) (Si,Al)12(O,N)16 (where M is at least one metal selected from the group consisting of Eu, Tb, Yb and Er, 0.05 less than (x+y) less than 0.3, 0.02 less than x less than 0.27 and 0.03 less than y less than 0.3) and having a structure such that Ca sites of Ca-xcex1-sialon are partially substituted by other metal M, at most 40 wt % of xcex2-sialon, and at most 30 wt % of unreacted silicon nitride.
8. The sialon type phosphor according to Item 7, wherein the chemical composition of the powder as a whole, is within a range defined by three compositional lines of Si3N4-a(M2O3.9AlN), Si3N4-b(CaO.3AlN) and Si3N4-c(AlN.Al2O3), where 4xc3x9710xe2x88x923 less than a less than 4xc3x9710xe2x88x922, 8xc3x9710xe2x88x923 less than b less than 8xc3x9710xe2x88x922 and 10xe2x88x922 less than c less than 10xe2x88x921.